Formation of a functional neural network requires the establishment of correct connections between the axons of presynaptic neurons and the dendrites of postsynaptic neurons. The MARCM (Mosaic Analysis with a Repressible Cell Marker) system allows unique positive labeling of homozygous mutant cells in mosaic tissues, providing a powerful tool to study a subset of neurons in the complex central nervous system. Recent lineage analysis of projection neurons in the Drosophila antennal lobe has uncovered a previously unanticipated role for precise dendritic targeting by postsynaptic neurons in determining connection specificity; that is, the antennal lobe projection neurons are pre-specified by lineage and birth order to project their dendrites to synapse with specific incoming olfactory receptor neuron axons. The molecular mechanism underlying Drosophila projection neuron precise dendritic targeting will be studied with the MARCM system. Candidate genes will be tested for their possible function in precise dendritic targeting. Furthermore, a forward genetic screen will be performed to identify novel genes regulating precise dendritic targeting. The research proposed here is likely to enable us to understand the logic underlying neural map formation.